Claims:We Claim:
1. An electroplating composition comprising zinc sulphate in an amount of about 250 g/L, zinc chloride in an amount of about 6 g/L, boric acid in an amount of about 30 g/L, titania or silica nanoparticles in an amount of about 0.5-5 g/L, and a surfactant, wherein the electroplating composition has a pH of about 3.5.
2. The electroplating composition as claimed in claim 1, wherein the titania or silica nanoparticles are pre-treated with said surfactant.
3. The electroplating composition as claimed in claim 1 or 2, wherein:
a. the titania nanoparticles are present in an amount of about 0.5 g/L or 1 g/L and said surfactant is a non-ionic surfactant,
b. the titania nanoparticles are present in an amount of about 5 g/L and said surfactant is a cationic or an anionic surfactant, or
c. the titania nanoparticles are present in an amount of about 1 g/L and said surfactant is a cationic surfactant.
4. The electroplating composition as claimed in claim 3, wherein said non-ionic surfactant is Triton X-100, said cationic surfactant is cetyl trimethyl ammonium bromide (CTAB), and said anionic surfactant is sodium dodecyl sulfate (SDS).
5. The electroplating composition as claimed in claim 1 or 2, wherein:
a. the silica nanoparticles are present in an amount of about 2.5 g/L and said surfactant is an anionic or a non-ionic surfactant, or
b. the silica nanoparticles are present in an amount of about 0.5 g/L and said surfactant is an anionic surfactant.
6. The electroplating composition as claimed in claim 5, wherein said non-ionic surfactant is sodium hexamethyl phosphate (SHMP) and said anionic surfactant is sodium dodecyl sulfate (SDS).
7. A method for preparing the electroplating composition as claimed in any one of claims 1-6, comprising:
a. adding the surfactant to demineralized water followed by heating to about 30-40?, stirring and adjusting pH to obtain a first solution;
b. adding the titania or silica nanoparticles to the first solution followed by stirring for about 20-30 minutes and ultrasonication for about 1 hour to obtain a second solution;
c. adding zinc sulphate, zinc chloride, and boric acid to demineralized water followed by stirring for about 1 to 1.5 hours and adjusting pH to obtain a third solution;
d. adding the second solution to a part of the third solution followed by adjusting pH and stirring for about 30 minutes to obtain a fourth solution;
e. adding the fourth solution to the remaining part of the third solution to obtain a fifth solution followed by adjusting volume and pH of the fifth solution; and
f. stirring the fifth solution for about 24 hours followed by ultrasonication for about 30 minutes to obtain the electroplating composition.
8. The method as claimed in claim 7, wherein the stirring rate is about 250-350 rpm.
9. The method as claimed in claim 7 or 8, wherein the ultrasonication is performed at a frequency of about 25-35 kHz.
10. A method for depositing the electroplating composition as claimed in any one of claims 1-6 on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. depositing the electroplating composition on the steel substrate at a constant current with a current density of about 170-190 mA/cm2 and at a stirring rate of about 250-350 rpm to provide a steel substrate comprising a zinc-titania (Zn- TiO2) or a zinc-silica (Zn-SiO2) coating.
11. The method as claimed in claim 10, wherein the current density is about 180 mA/cm2.
12. The method as claimed in claim 10 or 11, wherein the Zn-TiO2 coating deposited by the method comprises about 1.9-2.4 % by weight of titania.
13. The method as claimed in any one of claims 10-12, wherein the Zn-TiO2 coating provided by the method exhibits a corrosion current density of about 0.6-0.7 µA/cm2.
14. The method as claimed in any one of claims 10-12, wherein the method provides a deposition rate of about 3 - 3.5 µm/min.
15. The method as claimed in claim 10 or 11, wherein the Zn-SiO2 coating deposited by the method comprises about 3.6-4 % by weight of silica.
16. The method as claimed in any one of claims 10, 11, or 15, wherein the Zn-SiO2 coating provided by the method exhibits a corrosion current density of about 2.3-2.9 µA/cm2.
17. A method for depositing the electroplating composition as claimed in any one of claims 1-6 on a steel substrate, comprising:
a. providing the steel substrate as a cathode;
b. depositing the electroplating composition on the steel substrate by employing a pulsed current with an average current density of about 180 mA/cm2, a duty cycle of about 50-75% and a frequency of about 25-200Hz to provide a steel substrate comprising a zinc-titania (Zn-TiO2) or a zin-silica (Zn-SiO2) coating.
18. The method as claimed in claim 17, wherein the pulsed current has a duty cycle of about 75% and a frequency of about 25 Hz.
19. The method as claimed in claim 17 or 18, wherein the Zn-TiO2 coating deposited by the method comprises about 1.9-2% by weight of titania.
20. The method as claimed in any one of claims 17-19, wherein the Zn-TiO2 coating provided by the method exhibits a corrosion current density of about 0.6-0.7 µA/cm2.
21. The method as claimed in claim 17, wherein the pulsed current has a duty cycle of about 50% and a frequency of about 200 Hz.
22. The method as claimed in claim 17 or 21, wherein the Zn-SiO2 coating deposited by the method comprises about 2.8-3.9 % by weight of silica.
23. The method as claimed in any one of claims 17, 21, and 22, wherein the Zn-SiO2 coating provided by the method exhibits a corrosion current density of about 1.8 - 2.5 µA/cm2.
24. A steel substrate comprising a zinc-titania (Zn-TiO2) coating, wherein the coating comprises about 1.9-2.4% by weight of titania.
25. The steel substrate as claimed in claim 24, wherein the coating exhibits a corrosion current density of about 0.6-0.7 µA/cm2.
26. A steel substrate comprising a zinc-silica (Zn-SiO2) coating, wherein the coating comprises about 2.5-5% by weight of silica.
27. The steel substrate as claimed in claim 26, wherein the coating exhibits a corrosion current density of about 1.8-2.9 µA/cm2

, Description:TECHNICAL FIELD
The present disclosure relates to the field of electroplating. Particularly, the present disclosure relates to electroplating compositions comprising zinc and chemically treated titania or silica particles, methods of preparing them, direct and pulsed current methods of depositing these electroplating compositions on steel substrates and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Hot dip galvanised (zinc coating) steel is the most popular coated steel product used in automotive segment. The conventional zinc coating can be modified to zinc alloy coatings to further improve the corrosion resistance properties. Electrodeposition technology offers higher flexibility than hot dip technology to coat steel with a variety of zinc alloys which can offer a higher corrosion resistance along with a better control over coating thickness (much lower thickness is possible), superior surface finish and possibility of only one-sided coating. However, there is a lot of scope to introduce special functionalities in the coating as demanded by the automotive segments. At present, the functionalities like self-passivation and self-lubrication properties are achieved through an additional inorganic or organic coating over and above the zinc metallic coating.

Oxides are an interesting category of materials that can be used as reinforcements for composite coatings. Oxides like titania, silica, alumina are known to have corrosion resistance properties due to the formation of a protective oxide layer that prevents further corrosion. These oxide particles may be added to the plating bath as nanoparticles, and further co-deposited with a metal or alloy matrix. Incorporation of oxide particles in the coating has several advantages. The main challenge for incorporation of oxide reinforcement particles in the coating is the dispersion and stability of these oxide particles in the electroplating bath. Nanoparticles have a high specific surface area; hence they tend to agglomerate and settle down in the bath. To overcome this phenomenon, addition of different surfactants has been attempted to disperse the reinforcement particles in an efficient way. Various studies have been carried out to study the dispersion of TiO2 and SiO2 particles with and without surfactants.

In earlier works, Zn has been studied for manufacturing metal matrix composite coatings with other reinforcements in terms of oxides, nitrides, polymeric particles and carbonaceous incorporations. Zn along with other metal matrix based composite coatings has been studied in terms of the design of the deposition line for shaped components. Metallic matrix of Ag, Au, Cu, Co, Cr, Ni, Fe, Pb, Pd, Pt, Rh, Ru, Sn, V, W and Zn with alloying of alloying elements selected from C, P, S and Si with oxide powders of Al, Co, Cu, In, Mg, Ni, Si, Sn, V, and Zn; nitrides of Al, B and Si; C (graphite or diamond); carbides of B, Cr, Bi, Si, W; and organic materials such as PTFE and polymer spheres. Properties like wear resistance, corrosion resistance, abrasion resistance, and the like, materials such as silicon carbide, aluminium oxide, tungsten carbide, titanium carbide, zirconium oxide, boron carbide, chromium carbide, iron oxide, thorium oxide, uranium oxide, rare earth oxides, or diamonds and for lubrication molybdenum disulphide have been studied through DC deposition [2]. Zinc based composite coating has also been studied with SiC in alkaline medium for improved wear resistance, though the deposition kinetics of the composite coating was a challenge being operated under an alkaline pH [3]. It has been reported that corrosion current (Icorr) values decrease as the TiO2 content in the coating increases. Since the corrosion current density, Icorr, represents the corrosion rate of the metallic materials, coating with the highest weight percentage of TiO2 (4 wt-%) are expected to provide the lowest rate of corrosion [4]. Under direct current (DC) conditions, TiO2 content in the coating gets reduced with an increase in the TiO2 concentration in the electrolyte. With the increase of applied current and TiO2 concentration of the electrolyte leads to a fine microstructure with corrosion currents as low as 2.7 µA/cm2. Under pulsed current conditions, incorporation rate of nanoparticles tends to increase with increasing pulse frequency and decreasing current density. Incorporation of TiO2 increases microhardness of the zinc coatings by improving deposit structure. According to Guglielmi model, particles at lower current density get adsorbed on the cathode surface by weak van der Waals forces and particles get adsorbed at higher current density on the surface by Coulombic forces [5]. Zinc-based composite coatings are the natural self-healing and wear resistant properties of zinc are enhanced. The previously studied composite coatings Zn-SiO2 coatings show better corrosion resistance than the pure zinc coating. The industrial electro galvanization of steel sheets is usually carried out at a low pH of approximately 2 and SiO2 is stable under these conditions. However, the co-deposition of silica is very low at this pH. Several reasons for this have been suggested. At low pH, the zeta (?) potential of silica is close to zero. This leads to the agglomeration of the particles owing to the compression of the diffuse double layer surrounding them because of the high ionic strength [6]. However, no correlation was observed between the ? potential and colloidal stability in some studies [7]. Since the ? -potential is an electro-kinetic quantity, it does not always correlate with colloidal stability and incorporation tendencies, which are governed by the static charge distribution around the particles. Moreover, some authors claim that electrostatic interaction with the surface is crucial for incorporation [6], i.e. a net positive charge is required for incorporation into the negatively polarized electrode, which is not the case for silica particles in the plating bath. However, the experimental results are not fully conclusive and a clear proof of the possible role of electrostatic interaction has not been provided yet [7]. A general observation is made that a significant higher particle content of the layers is deposited from neutral and alkaline electrolytes compared to the acidic electrolyte. If it is taken into consideration that the particles are negatively charged in these electrolytes this finding is difficult to rationalize – how a negatively charged particle be incorporated more easily than a positively charged one [8]. Without detailed investigations of the interaction forces of the particles and the electrode, the underlying mechanisms can only be speculated. The negative charge of the particles is due to a preferential adsorption of anions. One possible explanation is that the negatively charged particles are attracted by the double layer of the substrate. Under electroplating conditions this double layer should bear positive excess charges which interact with the negatively charged particles [9]. The particles will also have a double layer around them, but this will be deformed. The particle is moving towards the electrode and the centre of its ionic shell lags a little bit behind. In the strong electric field of the double layer the hull of adsorbed anions is stripped off and the particle is incorporated into the growing metal matrix. This argument does not imply that electrodeposition is completely governed by electrostatic forces. The proposed mechanism helps to rationalize the experimental results for negatively charged particles being incorporated in higher amounts than positively charged ones [9].

Dispersion studies
Stability of oxide particles in aqueous phase is of significant importance in particle processing, dispersion and applications [11], [12]. Physical properties of particle suspensions are dependent on the behaviour of aqueous dispersions, which is especially reactive to the electrical and ionic structure of the particle–liquid interface [13], [14]. Oxide nanoparticle’s surface can be modified by functionalization [15], [16]. The long-term stability of the nanofluid can be improved by addition of functionalized nanoparticles. This helps maintain low viscosity increase without any contamination, leading to a good fluidity of nanofluids [17], [18], [19]. According to Liao et al [20], surfactants may influence size and form of TiO2 nanoparticles during preparation that have a significant impact on the optical and electronic properties of obtained nanoparticles. These changes may cause constriction of band gap and increase of photocatalytic activity of TiO2 nanoparticles.

It was reported by Ahmad et al. that particles dispersion depends on the type of surfactant [21]. Surfactants can be categorized into cationic, anionic and non-ionic. Due to the negative charge of SiO2 nanoparticles in aqueous suspensions, the cationic surfactants are of interest as additives. There are two groups of cationic surfactants: strong polyelectrolytes, for which the degree of ionization is independent of the solution pH, and weak polyelectrolytes, for which the degree of ionization is determined by the solution pH [22].

Deposition Studies
Among the nanomaterials, titanium dioxide (TiO2) has gained more attention in recent years. Successful results have been reported on the co-deposition of TiO2 with Ni, Cu, Zn and Ag metals [23]. Studies on electrodeposition of nanocomposite coatings have been directed towards the determination of optimum conditions for their synthesis, i.e. current density, temperature, pH, particle concentration and surfactant [24], [25], [26]. Ohko et al. reported the corrosion protective properties of TiO2 on 304 stainless steel [27].

The electro co-deposition of suspended inert particles into a growing metal matrix seems to offer a possible solution to this problem and has attracted research interest due to their unique functional properties such as improved corrosion [28] and wear resistance [29], lubricity [30]. The electro co-deposition of Zn-SiO2 has been reported with the effect of SiO2 colloid on the electrodeposition of zinc–iron group metal alloy composites [31]. According to this study, SiO2 has a negative zeta potential, and therefore its electrophoresis to the cathode cannot be expected. However, a phenomenon of induced co-deposition was observed, since SiO2 increased the percentage of Fe, Co, or Ni in the alloys and the iron group metal cations simultaneously accelerated the co-deposition of SiO2 [31]. For enhanced co-deposition of ceramic particles, the effect of various types of electrodeposition parameters have previously been reported such as agitation [32], surfactants in the bath [32].

Although several studies have been attempted to provide Zn composite coatings with titania/silica particles reinforcement, there is still a need in the art to provide a comprehensive coating solution where multifunctional requirements (e.g., self-passivation and self-lubrication that are currently achieved through an additional inorganic or organic coating over and above the zinc metallic coating) of the automotive industry can be achieved through a single coating system. The present disclosure attempts to address this need.
STATEMENT OF THE DISCLOSURE
The present disclosure relates to an electroplating composition comprising zinc sulphate in an amount of about 250 g/L, zinc chloride in an amount of about 6 g/L, boric acid in an amount of about 30 g/L, titania or silica particles in an amount of about 0.5-5 g/L, and a surfactant, wherein the electroplating composition has a pH of about 3.5.

The present disclosure also relates to a method for preparing the electroplating composition described herein, comprising: a) adding the surfactant to demineralized water followed by heating to about 30-40?, stirring and adjusting pH to obtain a first solution; b) adding the titania or silica nanoparticles to the first solution followed by stirring for about 20-30 minutes and ultrasonication for about 1 hour to obtain a second solution; c) adding zinc sulphate, zinc chloride, and boric acid to demineralized water followed by stirring for about 1 to 1.5 hours and adjusting pH to obtain a third solution; d) adding the second solution to a part of the third solution followed by adjusting pH and stirring for about 30 minutes to obtain a fourth solution; e) adding the fourth solution to the remaining part of the third solution to obtain a fifth solution followed by adjusting volume and pH of the fifth solution; and f) stirring the fifth solution for about 24 hours followed by ultrasonication for about 30 minutes to obtain the electroplating composition.

The present disclosure provides direct current and pulsed current methods for depositing the electroplating compositions on steel substrates.

The direct current method for depositing the electroplating composition on a steel substrate, comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 170-190 mA/cm2 and at a stirring rate of about 250-350 rpm to provide a steel substrate comprising a zinc-titania (Zn-TiO2) or a zinc-silica (Zn-SiO2) coating.

The pulsed current method for depositing the electroplating composition on a steel substrate, comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate by employing a pulsed current with an average current density of about 180 mA/cm2, a duty cycle of about 50-75% and a frequency of about 25-200Hz to provide a steel substrate comprising a Zn-TiO2 or a Zn-SiO2 coating.

The present disclosure further relates to a steel substrate comprising a Zn-TiO2 coating, wherein the coating comprises about 1.9-2.4% by weight of titania. In some embodiments, the Zn-TiO2 coatings comprising about 1.9-2.4% by weight of titania exhibit a corrosion current density of about 0.6-0.7 µA/cm2.

The present disclosure also relates to a steel substrate comprising a Zn-SiO2 coating, wherein the coating comprises about 2.5-5% by weight of silica. In some embodiments, the Zn-SiO2 coatings comprising about 2.5-5% by weight of silica exhibit a corrosion current density of about 1.8-2.9 µA/cm2.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows the schematic of an exemplary method of preparing the electroplating composition.

Figure 2 shows the transmission electron microscopy (TEM) images of TiO2 particles (left panel) and the X-ray diffraction (XRD) pattern of TiO2 particles.

Figure 3 shows the TEM images of SiO2 particles (left panel) and the XRD pattern of SiO2 particles.

Figure 4 shows the zeta potential values of SiO2 particles (left panel) and TiO2 particles (right panel).

Figure 5 shows corrosion currents of Zn-TiO2 composite coatings obtained by the direct current method at varying titania nanoparticles concentration in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 6 shows corrosion currents of Zn-TiO2 composite coatings obtained by the pulsed current method (P1 pulse parameters) at varying concentrations of titania nanoparticles in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 7 shows corrosion currents of Zn-TiO2 composite coatings obtained by the pulsed current method (P2 pulse parameters) at varying titania nanoparticles concentration in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 8 shows the top surface morphology (upper panel) and the cross-section morphology (lower panel) of the Zn-TiO2 composite coatings.

Figure 9 shows corrosion currents of Zn-SiO2 composite coatings obtained by the direct current method at varying concentrations of silica nanoparticles in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 10 shows corrosion currents of Zn-SiO2 composite coatings obtained by the pulsed current method (P1 pulse parameters) at varying concentrations of silica nanoparticles in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 11 shows corrosion currents of Zn-SiO2 composite coatings obtained by the pulsed current method (P2 pulse parameters) at varying concentrations of silica nanoparticles in the presence of no surfactant (physical dispersion), a cationic surfactant, an anionic surfactant and a non-ionic surfactant.

Figure 12 shows the top surface morphology (upper panel) and the cross-section morphology (lower panel) of the Zn-SiO2 composite coatings.

Figure 13 shows corrosion currents of the benchmark/commercially used coating (Zn-Ni passivated coating), pure Zn coating, Zn coatings with additives, Zn coatings obtained by physical dispersion, and the surfactant-treated Zn-TiO2 and Zn-SiO2 coatings.

DETAILED DESCRIPTION OF THE DISCLOSURE
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Reference throughout this specification to “some embodiments”, “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

As used herein, the term “electroplating composition” refers to an electroplating bath comprising electrolytes (Zn salts) and surfactant-treated titania or silica nanoparticles. In some embodiments, the size of the nanoparticles ranges from about 20 nm to about 30 nm, including values and ranges thereof.

The term “titania” as used herein refers to titanium dioxide (TiO2). The term “silica” as used herein refers to silicon dioxide (SiO2).

The term “about” as used herein encompasses variations of +/-5% and more preferably +/-2.5%, as such variations are appropriate for practicing the present invention.

The present disclosure provides electroplating compositions for depositing Zn-titania (Zn-TiO2) or Zn-silica (Zn-SiO2) coatings on steel substrates. The electroplating compositions of the present disclosure provide a single coating solution for steel substrates and obviate the need of addition organic or inorganic coatings that are currently employed to provide various functionalities. Further, the present disclosure provides an improved method for preparing electroplating compositions comprising zinc salts and chemically treated, i.e., surfactant-treated, titania or silica nanoparticles. The present disclosure also provides methods for depositing/electroplating said compositions on steel substrates by a direct current (DC) method and a pulsed current method. The coatings of the present disclosure show improved deposition kinetics, improved surface microstructure, higher titania or silica content, and/or improved corrosion resistance.

In some embodiments, the present disclosure provides an electroplating composition comprising zinc sulphate in an amount of about 250 g/L, zinc chloride in an amount of about 6 g/L, boric acid in an amount of about 30 g/L, titania or silica nanoparticles at a concentration of about 0.5-5 g/L, and a surfactant, wherein the electroplating composition has a pH of about 3.5.

In the electroplating compositions of the present disclosure, titania or silica nanoparticles are pre-treated with a surfactant prior to mixing them with the rest of the components (zinc sulphate, zinc chloride, and boric acid) to enhance dispersion of the nanoparticles in the composition. The surfactant employed in the present disclosure can be a non-ionic, cationic, or an anionic surfactant.

In some embodiments, the non-ionic surfactant is selected from Triton X-100 (2-[4-(2,4,4-trimethylpentan-2-yl)phenoxy]ethanol); polyoxyethylene cetyl ethers such as BRIJ-52 (Polyoxyethylene (2) cetyl ether) or BRIJ-58 (Polyoxyethylene (20) cetyl ether), polysorbates such as TWEEN 20, TWEEN 50, TWEEN 68, sodium hexametaphosphate (SHMP), or a combination thereof. In some embodiments, the non-ionic surfactant is Triton X-100 (TX-100) or SHMP. In some embodiments, TX-100 is employed for titania-containing electroplating compositions in an amount of about 0.05 ml/L. In some embodiments, SHMP is employed for silica-containing electroplating compositions at a silica to SHMP ratio of 2:1. In some embodiments, SHMP is employed for silica-containing electroplating compositions in an amount of about 1 g/L.

In some embodiments, the cationic surfactant is selected from cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), trietahnoalamine (TEA), benzalkonium chloride (ADBAC), azithro benzyl trimethylammonium bromide (AZTAB), or a combination thereof. In some embodiments, the cationic surfactant is CTAB for both for titania and silica-containing electroplating compositions. In some embodiments, the ratio of titania/silica to CTAB is 1:2. In some embodiments, CTAB is employed in an amount of about 2 g/L.

In some embodiments, the anionic surfactant is selected from sodium dodecyl sulphate (SDS, also known as sodium lauryl sulphate (SLS)), sodium dodecyl benzene sulphonate (SDBS), sodium benzoic sulphonate (SBS), lithium dodecyl sulphate (LDS), ammonium lauryl ether sulfate (ALES), or a combination thereof. In some embodiments the anionic surfactant is SDS for both for titania and silica-containing electroplating compositions. In some embodiments, the ratio of titania to SDS is 1:1. In some embodiments, SDS is employed in an amount of about 1 g/L for titania-containing compositions and about 0.1 g/L for silica-containing compositions.

Titania or silica nanoparticles are present in the electroplating composition at a concentration of about 0.5-5 g/L, including values and ranges therebetween. For example, in some embodiments, titania or silica nanoparticles are present in the electroplating composition at a concentration of about 0.5, 0.6, 0.7, 0.75, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, or 5 g/L, including values therebetween. In some embodiments, titania or silica nanoparticles are present in the electroplating composition at a concentration of about 0.5-4.5, 0.5-4, 0.5-3.5, 0.5-3, 0.5-2.5, 0.5-2, 0.5-1.5, 0.5-1, 1-5, 1-4.5, 1-4, 1-3.5, 1-3, 1-2.5, 1-2, 1.5-5, 1.5-4.5, 1.5-4, 1.5-3.5, 1.5-3, 1.5-2.5, 2-5, 2-4.5, 2-4, 2-3.5, 2-3, 2.5-5, 2.5-4.5, 2.5-4, 2.5-3.5, 3-5, 3-4.5, 3-4, 3.5-5, 3.5-4.5, or 4-5 g/L, including values and ranges therebetween.

In some embodiments, the electroplating composition comprises about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and:
(i) about 0.5 g/L or 1 g/L titania nanoparticles pre-treated with a non-ionic surfactant (e.g., 0.05 ml/L of Triton X-100);
(ii) about 5 g/L titania nanoparticles pre-treated with a cationic (e.g., 2 g/L CTAB) or an anionic surfactant (1 g/L SDS); or
(iii) about 1 g/L titania nanoparticles pre-treated with a cationic surfactant (e.g., 2 g/L CTAB).

In some embodiments, the electroplating composition comprises about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and:
(i) about 2.5 g/L silica nanoparticles pre-treated with an anionic (e.g., 0.1 g/L SDS) or a non-ionic surfactant (e.g., 1 g/L SHMP); or
(ii) about 0.5 g/L silica nanoparticles pre-treated with an anionic surfactant (0.1 g/L SDS).

In some embodiments, the electroplating composition comprising about 1 g/L titania nanoparticles treated with a non-ionic surfactant like Triton X-100 provides a Zn-TiO2 coating comprising about 2.3-2.4% by weight of titania and exhibiting a corrosion current of about 0.6-0.7 µA/cm2.

In some embodiments, the electroplating composition comprising about 1 g/L titania nanoparticles treated with a cationic surfactant like CTAB provides a Zn-TiO2 coating comprising about 1.9-2% by weight of titania and exhibiting a corrosion current of about 0.6-0.7 µA/cm2.

In some embodiments, the electroplating composition comprising about 2.5 g/L silica nanoparticles treated with a non-ionic surfactant like SHMP provides a Zn-SiO2 coating exhibiting a corrosion current of about 2.3-2.8 µA/cm2.

In some embodiments, the electroplating composition comprising about 2.5 g/L silica nanoparticles treated with an anionic surfactant like SDS provides a Zn-SiO2 coating comprising about 3.6 wt% silica and exhibiting a corrosion current of about 2.8-2.9 µA/cm2.

In some embodiments, the electroplating composition comprising about 0.5 g/L silica nanoparticles treated with an anionic surfactant like SDS provides a Zn-SiO2 coating exhibiting a corrosion current of about 2.4-2.5 µA/cm2. In some embodiments, the electroplating composition comprising about 2.5 g/L silica nanoparticles treated with an anionic surfactant like SDS provides a Zn-SiO2 coating exhibiting a corrosion current of about 1.8-1.9 µA/cm2.

Also provided herein is a method of preparing the electroplating compositions of the present disclosure. In some embodiments, the method broadly comprises preparing the surfactant-treated titania/silica solution and ultrasonicating this solution; preparing an electrolyte solution comprising boric acid, zinc sulphate, and zinc chloride; adding the ultrasonicated surfactant-treated titania/silica solution to a part of the electrolyte solution and stirring the mixture thoroughly; adding the mixture to the rest of the electrolyte solution followed by stirring and ultrasonication to obtain the electroplating composition. In this method, titania/silica particles are first treated with a surfactant and then the surfactant comprising the titania/silica particles is added to the electrolyte. The inventors found that if titania/silica particles and a surfactant were added directly to the electrolyte, the titania/silica nanoparticles got agglomerated due to unavailability of the surfactant which interacted with the metal ions of the electrolyte instead of the titania/silica particles. Therefore, to disperse the titania/silica nanoparticles better, the inventors first emulsified the particles in a small volume of a surfactant solution prepared in water where the particles get a chance to interact with the surface modifying organic molecules of the surfactant which cover the nanoparticles to impart the designated charges. This small volume of the operating environment helped disperse the nanoparticles and then the emulsion/surfactant solution containing the nanoparticles was added to the main electrolyte.

In some embodiments, the method for preparing an electroplating composition of the present disclosure comprises: a) adding the surfactant to demineralized water followed by heating to about 30-40?, stirring and adjusting pH to obtain a first solution; b) adding titania or silica nanoparticles to the first solution followed by stirring for about 20-30 minutes and ultrasonication for about 1 hour to obtain a second solution; c) adding zinc sulphate, zinc chloride, and boric acid to demineralized water followed by stirring for about 1 to 1.5 hours and adjusting pH to obtain a third solution; d) adding the second solution to a part of the third solution followed by adjusting pH and stirring for about 30 minutes to obtain a fourth solution; e) adding the fourth solution to the remaining part of the third solution to obtain a fifth solution followed by adjusting volume and pH of the fifth solution; and f) stirring the fifth solution for about 24 hours followed by ultrasonication for about 30 minutes to obtain the electroplating composition.

First, a surfactant is added to demineralized (DM) water and is dissolved in the DM water by heating water to about 30-40? and stirring. The pH of the surfactant solution is adjusted to 3.5. Titania or silica particles are added to the surfactant solution, the solution is stirred for about 20-30 minutes at a speed of about 250-350 rpm and ultrasonicated for about 1 hour.

An electrolyte solution is prepared separately by adding boric acid, zinc sulphate, and zinc chloride in this order to DM water. Zn salts and boric acid are dissolved by stirring at a speed of about 250-350 rpm for about 1 to 1.5 hours and the pH is adjusted to 3.5.

The ultrasonicated surfactant solution is added to a portion of the electrolyte, pH is adjusted and the solution is stirred for about 30 minutes. This solution is added to the rest of the electrolyte solution, the volume and pH are adjusted. The electrolyte containing the surfactant and titania/silica particles is stirred for about 24 hours and ultrasonicated for about 30 minutes to obtain the electroplating composition.

In the above method, stirring is carried out at a speed of about 250-350 rpm such as about 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, or 350 rpm and the ultra-sonication is carried out at a frequency of about 25-35 kHz. In some embodiments, the ultra-sonication is carried out at a frequency of about 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, or 35 kHz. In some embodiments, the ultra-sonication is carried out at a frequency of about 28-32 kHz. In an exemplary embodiment, the ultra-sonication is carried out at a frequency of about 30 kHz.

The present disclosure also provides methods for depositing the electroplating compositions described herein on steel substrates to provide substrates with zinc-titania (Zn-TiO2) or zinc-silica (Zn-SiO2) coatings. In some embodiments, the electroplating compositions are deposited using a direct current (DC) method. In some embodiments, the electroplating compositions are deposited using a pulsed current method.

Direct Current (DC) Deposition
In some embodiments, a method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; b) depositing the electroplating composition on the steel substrate at a constant current with a current density of about 170-190 mA/cm2 and at a stirring rate of about 250-350 rpm to provide a steel substrate comprising a Zn-TiO2 or a Zn-SiO2 coating.

The inventors have found that the electroplating compositions of the present disclosure, when deposited by employing a constant current having a current density of about 170-190 mA/cm2, provide a higher rate of deposition compared to commercially used Zn-Ni coatings. In some embodiments, the rate of deposition provided by the DC deposition method is about 3-3.5 µm/min, including values and ranges thereof. The rate of deposition of the commercial Zn-Ni coating is 1µm/min.

In some embodiments, the current density employed in the DC method of deposition is about 170, 175, 180, 185, or 190 mA/cm2, including values and ranges thereof. In some embodiments, the current density employed in the DC method of deposition is about 175-185 mA/cm2, including values and ranges thereof. In an exemplary embodiment, the current density for the DC deposition is about 180 mA/cm2.

In some embodiments, an electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 1 g/L titania nanoparticles treated with a non-ionic (e.g., Triton X-100) or a cationic (e.g., CTAB) surfactant and having a pH of about 3.5 is deposited on a steel substrate at a current density of about 180 mA/cm2 and a stirring rate of about 300 rpm to provide a steel substrate comprising a Zn-TiO2 coating. The inventors found that the electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 1 g/L titania nanoparticles treated with a non-ionic surfactant (e.g., Triton X-100) and having a pH of about 3.5 provides a single coating solution, when deposited by a direct current, to provide a coating with corrosion properties superior than the commercial Zn-Ni coating with a passivation.

In some embodiments, the DC method provides a Zn-TiO2 coating comprising about 1.9-2.4% by weight of titania and exhibiting a corrosion current of about 0.6-0.7 µA/cm2, including values and ranges thereof.

In some embodiments, an electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 2.5 g/L silica nanoparticles treated with a non-ionic (e.g., SHMP) or an anionic (e.g., SDS) surfactant and having a pH of about 3.5 is deposited on a steel substrate at a current density of about 180 mA/cm2 and a stirring rate of about 300 rpm to provide a steel substrate comprising a Zn-SiO2 coating. In some embodiments, the DC method provides a Zn-SiO2 coating comprising about 3.6-4% by weight of silica and exhibiting a corrosion current of about 2.3-2.9 µA/cm2.

Pulsed Current Deposition
In some embodiments, a method for depositing the electroplating composition on a steel substrate comprises: a) providing the steel substrate as a cathode; and b) depositing the electroplating composition on the steel substrate by employing a pulsed current with an average current density of about 180 mA/cm2, a duty cycle of about 50-75% and a frequency of about 25-200Hz to provide a steel substrate comprising a zinc-titania (Zn-TiO2) or a zinc-silica (Zn-SiO2) coating. The electroplating composition is stirred at a stirring rate of about 300 rpm during the deposition process.

In some embodiments, the pulsed current has a duty cycle of about 50, 55, 60, 65, 70, or 75% and a frequency of about 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195 or 200 Hz. In exemplary embodiments, the pulsed current has a duty cycle of about 75% and a frequency of about 25 Hz or a duty cycle of about 50% and a frequency of about 200 Hz.

In some embodiments, an electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 1 g/L titania nanoparticles treated with a cationic surfactant (e.g., CTAB) and having a pH of about 3.5 is deposited on a steel substrate by employing a pulsed current with an average current density of about 180 mA/cm2, a duty cycle of about 75% and a frequency of about 25Hz to provide a steel substrate comprising a Zn-TiO2 coating. The inventors found that the electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 1 g/L titania nanoparticles treated with a cationic surfactant (e.g., CTAB) and having a pH of about 3.5 provides a single coating solution, when deposited by a pulsed current having a duty cycle of about 75% and a frequency of about 25Hz, to provide a coating with corrosion properties superior than the commercial Zn-Ni coating with a passivation.

In some embodiments, the pulsed method provides a Zn-TiO2 coating comprising about 1.96% by weight of titania and exhibiting a corrosion current of about 0.6-0.7 µA/cm2.

In some embodiments, the pulsed method provides a deposition rate of about 3 to 3.5 µm/min for the titania-containing electroplating compositions.

In some embodiments, an electroplating composition comprising about 250 g/L zinc sulphate, about 6 g/L zinc chloride, about 30 g/L boric acid, and about 0.5 g/L or about 2.5 g/L silica nanoparticles treated with an anionic surfactant (e.g., SDS) and having a pH of about 3.5 is deposited on a steel substrate by employing a pulsed current with an average current density of about 180 mA/cm2, a duty cycle of about 50% and a frequency of about 200 Hz to provide a steel substrate comprising a Zn-SiO2 coating.

In some embodiments, the pulsed method provides a Zn-SiO2 coating comprising about 2.8-4.4% by weight of silica and exhibiting a corrosion current of about 1.8-2.9 µA/cm2, including values and ranges thereof. In some embodiments, the pulsed method provides a deposition rate of about 3 µm/min to 3.5 µm/min for the silica-containing electroplating compositions.

The present disclosure provides a steel substrate comprising a zinc-titania (Zn-TiO2) or zinc-silica (Zn-SiO2) coating.

In some embodiments, the steel substrate comprises a zinc-titania (Zn-TiO2) coating, wherein the coating comprises about 1.9-2.4% by weight of titania, including values and ranges therebetween. For example, in some embodiments, the steel substrate comprises a Zn-TiO2 coating comprising about 1.9, 2, 2.1, 2.2, 2.3, or 2.4% by weight of titania. In some embodiments, the steel substrates comprising about 1.9-2.4% by weight of titania exhibit a corrosion current density of about 0.6-0.7 µA/cm2, including values and ranges thereof. In some embodiments, the steel substrate comprising a Zn-TiO2 coating is obtained by the direct current method, wherein the coating comprises about 2.3% by weight of titania and has a corrosion current density of about 0.6 µA/cm2. In some embodiments, the steel substrate comprising a Zn-TiO2 coating is obtained by the pulsed current method, wherein the coating comprises about 1.96% by weight of titania and has a corrosion current density of about 0.64 µA/cm2.

In some embodiments, the steel substrate comprises a zinc-silica (Zn-SiO2) coating, wherein the coating comprises about 2.5-5% by weight of silica, including values and ranges therebetween and exhibit a corrosion current density of about 1.8-2.9 µA/cm2. For example, in some embodiments, the steel substrate comprises a Zn-SiO2 coating comprising about 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, or 5 % by weight of silica. In some embodiments, the steel substrate comprises a Zn-SiO2 coating comprising about 3.6-4% by weight of silica and having a corrosion current density of about 2.3-2.9 µA/cm2. In some embodiments, the steel substrate comprises a Zn-SiO2 coating comprising about 2.8-3.9% by weight of silica and having a corrosion current density of about 1.8-2.5 µA/cm2.
It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES

Example 1: Characterization of titania and silica nanoparticles
TiO2 particles sourced (from Aeroxide) as hydrophilic fumed titanium dioxide were characterised through the transmission electron microscopy (TEM) as shown in Figure 2 to check the particle size, agglomeration tendency and diffraction pattern. The particle size was found to be in the range of 30-50 nm. The phase of TiO2 (Figure 2) particles were analysed through X-ray diffraction (XRD) and detected to have a crystalline anatase structure with a very high specific surface area (BET) varying from 35 – 65 m2/g and very good thermal and chemical stability. The chemical composition of TiO2 is shown in Table 1.
Table 1 Chemical composition of TiO2 particles

SiO2 particles sourced (from Nano Research Lab) as amorphous silicon dioxide were characterised through the TEM as shown in Figure 3 to check the particle size, agglomeration tendency and diffraction pattern. The particle size was found to be in the range of 30-50 nm. The XRD of SiO2 nanoparticles showed an amorphous structure for the nanoparticles with a specific surface area (BET) of 200 – 600 m2/g, true Density of 2.5 g/cm3 and bulk Density of 0.10 g/cm3.

Example 2: Zeta Potential Measurement
TiO2 and SiO2 reinforcement particles were then studied for a chemical dispersion tendency through zeta potential measurement in 100 ml of DM water in the working range of pH (3 to 3.5) varying the surfactant concentration at 1% particle concentration, i.e., with 1 g of particles in 100 ml of water containing varying concentration of the surfactant and then compared with the self-dispersion zeta potential indicating the enhancement. It is known that a zeta value of more than ±30 mV provides a better dispersion of the particles. The electroplating bath has a pH of 3.5. It was observed that self-dispersed TiO2 (physically dispersed) has zeta potential more than +30 mV at 3.5 pH which got further improved with cationic and anionic surfactants giving higher zeta potentials making it very well dispersed. In a physical dispersion, SiO2 has a zeta value near zero which improved substantially with the help of cationic and anionic surfactants signifying enhanced dispersion tendency of the same in the working range of pH as shown in Figure 4. Non-ionic surfactants also improved the dispersion through steric hindrance and thus changes in zeta potential were not reflected.

Example 3: Preparation of the electroplating composition
The electroplating bath/composition containing 250 g/L zinc sulphate, 6 g/L zinc chloride, 30 g/L boric acid, 0.5-5 g/L titania or silica nanoparticles, and a surfactant was prepared as shown in Figure 1. Specifically, the surfactant was added to about 100-250 ml demineralized water depending on the solubility of the surfactant and the pH was adjusted to 3.5. The water was heated to 30-40? and stirred at 300 rpm to dissolve the surfactant. The entire amount of titania or silica nanoparticles were added to the surfactant solution and the solution was stirred for about 20 minutes. The titania/silica containing surfactant solution was ultrasonicated for about 1 hour and the pH was adjusted, if required.

To 500 ml of demineralized water, boric acid, zinc sulphate, and zinc chloride were added; the solution was stirred for about 1 to 1.5 hour; and the pH was adjusted to obtain the main electrolyte. 250 ml of the main electrolyte was taken in a separate beaker and stirring was continued for the rest of the electrolyte. The ultrasonicated titania/silica containing surfactant solution was added to 250 ml of the main electrolyte, the solution was stirred for 30 minutes, and the pH was adjusted to 3.5. The rest of the electrolyte was added to the electrolyte containing the ultrasonicated titania/silica containing surfactant solution. The volume of the solution was adjusted to 1 L, pH was adjusted to 3.5 and the stirring was continued for 24 hours. After 24 hours, the solution was ultrasonicated for 30 minutes to obtain the electroplating composition. The particle dispersion was achieved through the surfactant treatment of titania/silica nanoparticles, ultrasonication and magnetic stirring.

Example 4: Direct current (DC) deposition of Zn-TiO2 coating
Electroplating compositions comprising 250 g/L zinc sulphate; 6 g/L zinc chloride; 30 g/L boric acid; and 0.5, 1, 2.5, or 5 g/L titania particles pre-treated with 2 g/L CTAB, 1 g/L SDS, or 0.05 ml/L Triton X-100 were prepared. These compositions were deposited on steel substrates using direct current. The steel substrate was used as a cathode and pure Zn (99.5% pure) was used as an anode. Prior to deposition, the steel samples were degreased to remove surface oil and then dipped in a dilute HCl solution to remove any oxide film which might be present. The samples were rinsed in distilled water and the deposition was carried out. The electroplating compositions were deposited on the steel substrate at a constant current with a current density of 180 mA/cm2 and at a stirring rate of about 300 rpm to provide the steel substrate comprising a zinc-titania (Zn-TiO2) coating.

During deposition, the electrolyte was stirred at a constant rate of 300 rpm using a magnetic stirrer. The current was supplied through a potentiostat. After plating/deposition, the coated samples were rinsed with distilled water and dried.

Various characterizations were performed on the coated steel substrate. Scanning Electron Microscope (SEM) (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain wt.% of titania/silica in the coating at different current densities and potentiodynamic polarization test (Make: Gamry) was done to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation.

The potentiodynamic polarization test results showing corrosion current densities of Zn-titania composite coatings at different particle concentrations and with different surfactants are shown in Figure 5. The corrosion current was calculated using Gramry software (Version: 4.35) by Tafel extrapolation method. In Figure 5, for each particle concentration, the first bar on the left is for physical dispersion of the electroplating composition (i.e., no surfactant treatment), the second bar is for the cationic/CTAB-treated titania particles containing electroplating composition, the third bar is for the anionic/SDS-treated titania particles containing electroplating composition, and the fourth bar is for the non-ionic/Triton X-100-treated titania particles containing electroplating composition. Figure 5 shows that at lower concentrations of titania particles, the non-ionic surfactant (Triton X-100) has performed better in terms of Icorr (corrosion current density), whereas for cationic and anionic surfactant, the Icorr improved at higher concentration of particles due to the imparted surface charge on the surface of the particles prohibiting particle agglomeration. At 0.5 g/L titania concentration, the Icorr value for the Zn-TiO2 coating obtained from the electroplating composition comprising the non-ionic/Triton X-100 treated titania particles was 0.97 µA/cm2 whereas the Icorr value obtained for the physical dispersion was 3.83 µA/cm2. At 1 g/L titania concentration, the Icorr value for the Zn-TiO2 coating obtained from the electroplating composition comprising the non-ionic/Triton X-100 treated titania particles was the lowest - 0.6 µA/cm2 - whereas the Icorr value obtained for the physical dispersion was 1.12 µA/cm2. At 5 g/L titania concentration, the Icorr value for the Zn-TiO2 coating obtained from the electroplating composition comprising the cationic/Triton X-100 treated titania particles was 1.18 µA/cm2, 1.35 µA/cm2 for the physical dispersion, and 1.38 µA/cm2 for the electroplating composition comprising the anionic/SDS-treated titania particles. The corrosion current of the benchmark/commercial Zn-Ni passivated coating was 0.96 µA/cm2 (Figure 13).

The deposition kinetics and corrosion potential of the coatings were mostly in the similar range varying from 3 µm/min to 3.5 µm/min and -1.05V to -1.11V. It indicates the better dispersion of TiO2 particles in the presence of surfactants.

It was observed that the surfactant-treated TiO2 particles embedded in the metallic matrix were able to improve the corrosion properties without any secondary coatings and was even better than the benchmark that is the commercial Zn-Ni sample with passivation.

Example 5: Pulsed current deposition of Zn-TiO2 coating
The pulsed current deposition was performed galvanostatically using a Potentiostat (Make: AMETEK) in a two-electrode setup. The steel substrate was used as a cathode and pure Zn (99.5% pure) as an anode. Prior to deposition, the steel substrate was degreased to remove surface oil and then dipped in a dilute HCl solution to remove any oxide film which might be present. The substrate was rinsed in distilled water and the deposition was carried out. The electroplating composition was deposited on the steel substrate by employing a pulsed current having an average current density of about 180 mA/cm2 and two different duty cycles/frequency parameters. P1 represents pulse 1 with 50% duty cycle and 200Hz frequency and P2 represents pulse 2 with 75% duty cycle and 25Hz frequency.

During deposition, the electrolyte was stirred at a constant rate of 300 rpm using a magnetic stirrer. The current was supplied through a potentiostat. After plating/deposition, the coated substrates were rinsed with distilled water and dried.

Various characterizations were performed on the coated steel substrate. Scanning Electron Microscope (SEM) (Make: Nova-Nano) study was conducted to observe the coating morphology, SEM-EDS to obtain wt.% of titania/silica in the coating at different current densities and potentiodynamic polarization test (Make: Gamry) was done to obtain the corrosion potential (Ecorr) and corrosion current density (ICorr) values using Tafel extrapolation.
The corrosion current densities (Icorr) of Zn-TiO2 composite coatings deposited at P1 pulse parameters are shown in Figure 6 and the Icorr values for the coatings deposited at P2 pulse parameter are shown in Figure 7.

The Zn-TiO2 composite coatings deposited at the higher duty cycle and lower frequency showed superior corrosion resistance compared to the lower duty cycle and higher frequency at 1g/L titania particle concentration.

Corrosion current properties of Zn-TiO2 composite coatings deposited by the DC and pulsed method are shown below in Table 2. The best corrosion results, shaded in grey, were obtained with non-ionic dispersion and cationic dispersions at lower concentrations.

Table 2 Comparative Icorr (µA/cm2) results for Zn-TiO2 composite coating

The corresponding top surface morphologies for the Zn-TiO2 composite coatings with promising corrosion properties along with the TiO2 content in the coating were analysed. It was observed that the surfactant treatment has played a role to incorporate the TiO2 particles in the coatings. In Figure 8, upper panel, top surface morphologies of the most promising Zn-TiO2 composite coatings through the surfactant treatment of 0.5 g/l to 5 g/L of TiO2 in the electroplating bath are shown. It is clearly visible that the dispersion strategy for the particles can completely change the microstructure of the coating leading to good corrosion properties. It was observed that with the use of surfactants, more TiO2 got incorporated indicating more particle content in the coating. The mechanism of incorporating TiO2 particles changes with the surfactants along with the top surface morphologies. The surfactants act as grain refiners and change the activation sites of zinc nucleus and thus encourage a single or multiple selected orientation predominantly to enhance the properties along with the particles reinforcing into the coating. The cross-section images (Figure 8, lower panel) clearly show the embedded TiO2 particles in the coatings with completely different morphologies. A correlation is drawn for the TiO2 content and corrosion properties with the dispersion strategy as shown in Table 3.
Table 3 Correlation of TiO2 content and Icorr with the dispersion strategy
Dispersion Strategy TiO2 content (wt%) Icorr (µA/cm2)
Physical (without surfactant) 2.47 1.12
Cationic (CTAB) 1.96 0.64
Anionic (SDS) 2.58 1.38
Non-ionic (TX 100) 2.32 0.60

Example 6: Direct current (DC) and pulsed current deposition of Zn-SiO2 coating
Zn-SiO2 containing electroplating compositions were deposited by a direct current as described in Example 4 and various characterizations were performed.

For SiO2 particles, cetyl trimethyl ammonium bromide (CTAB) was the cationic surfactant, sodium dodecyl sulfate (SDS) was the anionic surfactant and sodium hexamethyl phosphate (SHMP) was the non-ionic surfactant. The deposition kinetics and corrosion potential of the coatings were mostly in the range of 3 µm/min to 3.5 µm/min and -1.05V to -1.11V.

Figure 9 shows the corrosion current values of the coatings obtained by the DC deposition of the surfactant-treated silica containing electroplating compositions. For the DC deposited coatings, Icorr values are better for the anionic and the non-ionic surfactant at 2.5 g/L titania concentration. Figures 10 and 11 show the corrosion current values of the coatings obtained by the P1 and P2 pulsed deposition of the surfactant-treated silica containing electroplating compositions. The anionic surfactant showed better Icorr values for the P1 pulsed deposition due to the imparted surface charge on the surface of the particles prohibiting particle agglomeration. The cationic and non-ionic surfactants did not work well for the P1 pulsed deposition whereas, all the surfactants deteriorated the Icorr properties in P2 deposition. The deposition kinetics and corrosion potential (Ecorr) of the coatings were mostly in the similar range signifying no impact of the concentration or the type of surface treatment of the particles.

Corrosion current properties of Zn-SiO2 composite coatings deposited by the DC and pulsed method are shown below in Table 4.

Table 4 Comparative Icorr (µA/cm2) results for Zn-SiO2 composite coating

The corresponding top surface morphologies for the Zn-SiO2 composite coatings with better corrosion properties along with the SiO2 content in the coating were analysed. It was observed that the surface treatment has played a role to incorporate the SiO2 particles in the coatings. In Figure 12, upper panel, top surface morphologies of the most promising Zn-SiO2 composite coatings are shown. It is clearly visible that the dispersion strategy of the particles can completely change the microstructure of the coating and affect the corrosion properties. It was observed that through surfactants, more SiO2 was incorporated indicating more particle content in the coating. The mechanism of incorporation of SiO2 particles changes with the surfactants along with the top surface morphologies. The cationic surfactant did not function well to disperse the SiO2 particles, but the anionic and non-ionic surfactants provided better dispersion at certain silica concentrations. These surfactants act as grain refiners and change the activation sites of zinc nucleus and thus encourage a single or multiple orientation predominantly to enhance the properties along with the particles reinforcing into the coating. The cross-section images (Figure 12, lower panel) clearly show the embedded SiO2 particles in the coating with completely different morphologies. A correlation is drawn for the SiO2 content and corrosion properties with the dispersion strategy as shown in Table 5.

Table 5 Correlation of SiO2 content and Icorr with the dispersion strategy
Dispersion Strategy SiO2 content (wt%) Icorr (µA/cm2)
Physical (without surfactant) 3.64 1.29
Anionic (SDS) 3.67 2.87
Anionic (SDS) 3.86 2.49
Anionic (SDS) 2.88 1.83
Non-ionic (SHMP) 4.32 2.58
Non-ionic (SHMP) 3.97 2.39

Example 7: Comparison of Zn-TiO2 coatings and Zn-SiO2 coatings with commercially employed coating
The most promising corrosion current values provided by the surfactant-treated Zn-TiO2 and Zn-SiO2 coatings were compared with those provided by the benchmark/commercially used coating (Zn-Ni passivated coating), pure Zn coatings, Zn coatings with additives, and Zn coatings obtained by physical dispersion of the baths. The results are shown in Figure 13. The Zn coating with surface treated TiO2 particles without any passivation has provided the single coating solution through DC (1 g/L TiO2 particles treated with a non-ionic surfactant like Triton X-100) or pulsed deposition (1 g/L TiO2 particles treated with a cationic surfactant like CTAB) and has given better corrosion property than the commercial Zn-Ni EG sample with Cr passivation.

References
[1] G. Palumbo, I. Brooks, J. McCrea, Process for electroplating metallic and metal matrix composite foils and microcomponents, EP1826294A1, 2002. http://info.sipcc.net/files/patent/fulltext/EP/200605/EP2099194A1/EP2099194A1.PDF.
[2] A.E. Grazen, Apparatus for producing composite electroplated articles, US3168457A, 1965.
[3] D. Xingshen, L. Yicheng, Preparation method of zinc-based composite coating, CN103911649A, 2014.
[4] T. Mokabber, S. Rastegari, H. Razavizadeh, Effect of electroplating parameters on properties of Zn – nano-TiO 2 composite coatings, 29 (2013) 41–46. doi:10.1179/1743294412Y.0000000077.
[5] M. Sajjadnejad, M. Ghorbani, A. Afshar, Microstructure-corrosion resistance relationship of direct and pulse current electrodeposited Zn-TiO 2 nanocomposite coatings, Ceram. Int. (2014). doi:10.1016/j.ceramint.2014.08.061.
[6] T.R. Khan, A. Erbe, M. Auinger, Electrodeposition of zinc – silica composite coatings?: challenges in incorporating functionalized silica particles into a zinc matrix, (2011). doi:10.1088/1468-6996/12/5/055005.
[7] T.J. Tuaweri, Influence of Process Parameters on the Cathode Current Efficiency of Zn/SiO2 Electrodeposition, Int. J. Mech. Eng. Appl. 1 (2013) 93. doi:10.11648/j.ijmea.20130105.11.
[8] X. Xia, I. Zhitomirsky, J.R. Mcdermid, Electrodeposition of zinc and composite zinc – yttria stabilized zirconia coatings, 9 (2008) 2632–2640. doi:10.1016/j.jmatprotec.2008.06.031.
[9] D. Thiemig, A. Bund, Characterization of electrodeposited Ni-TiO2 nanocomposite coatings, Surf. Coatings Technol. 202 (2008) 2976–2984. doi:10.1016/j.surfcoat.2007.10.035.
[10] A. Nag, A. Banerjee, A.N. Bhagat, The accomplishment of the novel range of operating parameters for DC and pulsed deposition with optimized bath composition for pure Zn, 202131012720, 2021.
[11] P. Fernández-Ibáñez, J. Blanco, S. Malato, F.J. De Las Nieves, Application of the colloidal stability of TiO2 particles for recovery and reuse in solar photocatalysis, Water Res. 37 (2003) 3180–3188. doi:10.1016/S0043-1354(03)00157-X.
[12] M. Kosmulski, pH-dependent surface charging and points of zero charge. III. Update, J. Colloid Interface Sci. 298 (2006) 730–741. doi:10.1016/j.jcis.2006.01.003.
[13] T. Imae, K. Muto, S. Ikeda, The pH dependence of dispersion of TiO2 particles in aqueous surfactant solutions, Colloid Polym. Sci. 269 (1991) 43–48. doi:10.1007/BF00654658.
[14] K. Cai, M. Frant, J. Bossert, G. Hildebrand, K. Liefeith, K.D. Jandt, Surface functionalized titanium thin films: Zeta-potential, protein adsorption and cell proliferation, Colloids Surfaces B Biointerfaces. 50 (2006) 1–8. doi:10.1016/j.colsurfb.2006.03.016.
[15] S.S. Shazali, S. Rozali, A. Amiri, M.N.M. Zubir, M.F.M. Sabri, M.Z. Zabri, Evaluation on stability and thermophysical performances of covalently functionalized graphene nanoplatelets with xylitol and citric acid, Mater. Chem. Phys. 212 (2018) 363–371. doi:10.1016/j.matchemphys.2018.03.040.
[16] S.S. Shazali, A. Amiri, M.N. Mohd Zubir, S. Rozali, M.Z. Zabri, M.F. Mohd Sabri, M. Soleymaniha, Investigation of the thermophysical properties and stability performance of non-covalently functionalized graphene nanoplatelets with Pluronic P-123 in different solvents, Mater. Chem. Phys. 206 (2018) 94–102. doi:10.1016/j.matchemphys.2017.12.008.
[17] Y. Li, J. Zhou, S. Tung, E. Schneider, S. Xi, A review on development of nanofluid preparation and characterization, Powder Technol. 196 (2009) 89–101. doi:10.1016/j.powtec.2009.07.025.
[18] A. Ghadimi, R. Saidur, H.S.C. Metselaar, A review of nanofluid stability properties and characterization in stationary conditions, Int. J. Heat Mass Transf. 54 (2011) 4051–4068. doi:10.1016/j.ijheatmasstransfer.2011.04.014.
[19] J. Kim, Y.S. Park, B. Veriansyah, J.D. Kim, Y.W. Lee, Continuous synthesis of surface-modified metal oxide nanoparticles using supercritical methanol for highly stabilized nanofluids, Chem. Mater. 20 (2008) 6301–6303. doi:10.1021/cm8017314.
[20] D.L. Liao, B.Q. Liao, Shape, size and photocatalytic activity control of TiO2 nanoparticles with surfactants, J. Photochem. Photobiol. A Chem. 187 (2007) 363–369. doi:10.1016/j.jphotochem.2006.11.003.
[21] N.R. Short, A. Abibsi, J.K. Dennis, Corrosion resistance of electroplated zinc alloy coatings, Trans. IMF. 67 (1991) 73–77. doi:https://doi.org/10.1080/00202967.1989.11870845.
[22] K. Kondo, Physical & Theoretical Chemistry fields Electrodeposition of Zinc-SiO2 composite, (1999) 2–5.
[23] J. Fustes, A. Gomes, M.I. Da Silva Pereira, Electrodeposition of Zn-TiO 2 nanocomposite films-effect of bath composition, J. Solid State Electrochem. 12 (2008) 1435–1443. doi:10.1007/s10008-007-0485-z.
[24] C. Guo, Y. Zuo, X. Zhao, J. Zhao, J. Xiong, The effects of electrodeposition current density on properties of Ni-CNTs composite coatings, Surf. Coatings Technol. 202 (2008) 3246–3250. doi:10.1016/j.surfcoat.2007.11.032.
[25] A. Bund, D. Thiemig, Influence of bath composition and pH on the electrocodeposition of alumina nanoparticles and copper, (2007) 345–351. doi:10.1007/s10800-006-9264-2.
[26] S. Spanou, E.A. Pavlatou, N. Spyrellis, Ni/nano-TiO2 composite electrodeposits: Textural and structural modifications, Electrochim. Acta. 54 (2009) 2547–2555. doi:10.1016/j.electacta.2008.06.068.
[27] Y. Ohko, S. Saitoh, T. Tatsuma, A. Fujishima, Photoelectrochemical Anticorrosion and Self-Cleaning Effects of a TiO[sub 2] Coating for Type 304 Stainless Steel, J. Electrochem. Soc. 148 (2001) B24. doi:10.1149/1.1339030.
[28] S. Hashimoto, M. Abe, THE CHARACTERIZATION OF ELECTRODEPOSITED Zn-SiO2 COMPOSITES BEFORE AND AFTER CORROSION TEST, Corros. Sci. 36 (1994) 2125–2137.
[29] C. Müller, M. Sarret, M. Benballa, ZnNi/SiC composites obtained from an alkaline bath, Surf. Coatings Technol. 162 (2003) 49–53. doi:10.1016/S0257-8972(02)00360-2.
[30] C. Filiâtre, C. Pignolet, A. Foissy, M. Zembala, P. Warszynski, Electrodeposition of particles at nickel electrode surface in a laminar flow cell, Colloids Surfaces A Physicochem. Eng. Asp. 222 (2003) 55–63. doi:10.1016/S0927-7757(03)00234-6.
[31] A. Takahashi, Y. Miyoshi, T. Hada, Effect of SiO2 Colloid on the Electrodeposition of Zinc-Iron Group Metal Alloy Composites, J. Electrochem. Soc. 141 (1994) 954–957. doi:10.1149/1.2054864.
[32] A. Hovestad, L.J.J. Janssen, Electrochemical codeposition of inert particles in a metallic matrix, J. Appl. Electrochem. 25 (1995) 519–527. doi:10.1007/BF00573209.